(133b) Liquid Phase Resistance to Mass Transfer in Structured Packing

Due to its low pressure drop and high mass transfer efficiency, structured packing becomes the most popular column internal in vacuum and low pressure distillation columns. Its application to the air separation industry has resulted in significant savings in energy consumption. Although many mass transfer efficiency models were developed by various research groups, the liquid phase resistance to mass transfer using structured packing in distillation is barely determined and little studied. This paper shows the significance of liquid phase resistance in predicting mass transfer efficiency, particularly for high purity systems. The predicted liquid phase resistance from various literature models1-3 are examined and compared. From limited literature data4, it was found that the liquid phase resistance to mass transfer is about 30% of the gas phase resistance for the C6/C7 system at 5 and 24 psia. The predicted liquid phase resistance from the Gualito model1 agrees well with experimental data. However, the Billet model2 significantly over predicts the liquid phase resistance. This discrepancy in the predicted liquid phase resistance from two models complicates the prediction of HETP as a function of the slope of the equilibrium line and the (L/V) ratio.